Magnetic memory with strain-assisted exchange coupling switch

ABSTRACT

A magnetic tunnel junction cell having a free layer and first pinned layer with perpendicular anisotropy, the cell including a coupling layer between the free layer and a second pinned layer, the coupling layer comprising a phase change material switchable from an antiferromagnetic state to a ferromagnetic state. In some embodiments, at least one actuator electrode proximate the coupling layer transfers a strain from the electrode to the coupling layer to switch the coupling layer from the antiferromagnetic state to the ferromagnetic state. Memory devices and methods are also described.

RELATED APPLICATIONS

This application claims priority to U.S. application Ser. No. 12/248,237filed Oct. 9, 2008 which is a provisional patent application No.61/086,873, filed Aug. 7, 2008. The entire disclosures of applicationSer. Nos. 12/248,237 and 61/086,873 are incorporated herein byreference.

BACKGROUND

Spin torque transfer technology, also referred to as spin electronics,combines semiconductor technology and magnetics, and is a more recentdevelopment. In spin electronics, the spin of an electron, rather thanthe charge, is used to indicate the presence of digital information. Thedigital information or data, represented as a “0” or “1”, is storable inthe alignment of magnetic moments within a magnetic element. Theresistance of the magnetic element depends on the moment's alignment ororientation. The stored state is read from the element by detecting thecomponent's resistive state.

The magnetic element, in general, includes a ferromagnetic pinned layerand a ferromagnetic free layer, each having a magnetization orientationthat defines the resistance of the overall magnetic element. Such anelement is generally referred to as a “spin tunneling junction,”“magnetic tunnel junction”, “magnetic tunnel junction cell”, and thelike. When the magnetization orientations of the free layer and pinnedlayer are parallel, the resistance of the element is low. When themagnetization orientations of the free layer and the pinned layer areantiparallel, the resistance of the element is high.

Application of spin torque transfer memory has a switching currentdensity requirement generally at 10⁶ to 10⁷ A/cm², which leads todifficulty in integrating with a regular CMOS process. It is desirableto reduce the switching current density significantly in order to make afeasible product.

Various bilayer heat-assisted media designs have been proposed thatattempt to lower the coercivity of the bilayer media and reduce theswitching field. However, there are major difficulties in implementingthese types of assisted switching. First, the reliability of the spintorque memory is a concern when heat assistance is utilized, due to theheat generated during switching; the assist temperature has thepotential to thermally degrade the magnetic layers of the spin torquememory. This high transition temperature may have adverse thermaleffects to the memory system, as the high power consumption neededduring write cycles produces large amounts of heat that need to bedissipated.

Other designs of assisted switching are needed.

BRIEF SUMMARY

The present disclosure relates to magnetic tunnel junction cells thatutilize spin torque and a strain induced by a phase change to assist inthe switching of the magnetization orientation of the free layer of themagnetic tunnel junction cell. The magnetic memory unit, which includesthe magnetic tunnel junction cell, can be utilized in a memory array.

In one particular embodiment, this disclosure describes a magnetictunnel junction cell comprising a first ferromagnetic pinned layer, aferromagnetic free layer, and a non-magnetic barrier layer therebetween.The first pinned layer and the free layer each have an out-of-planemagnetization orientation. The cell includes a second ferromagneticpinned layer and a coupling layer between the second pinned layer andthe free layer. The coupling layer comprises a phase change materialswitchable from an antiferromagnetic state to a ferromagnetic state.

In another particular embodiment, this disclosure describes a memorydevice comprising a magnetic tunnel junction cell including a couplinglayer between a second pinned layer and the free layer, the couplinglayer comprising a phase change material switchable from anantiferromagnetic state to a ferromagnetic state. The memory devicesincludes a first electrode and a second electrode electrically connectedto the magnetic tunnel junction cell to pass a spin currenttherethrough, at least one actuator electrode proximate the couplinglayer, and a voltage source electrically connected to the at least oneactuator electrode.

In yet another particular embodiment, this disclosure describes a methodof switching a memory device. The method includes switching a couplinglayer in a magnetic tunnel junction cell from its antiferromagneticstate to its ferromagnetic state by applying a voltage to an actuatorelectrode and creating a strain in the actuator electrode, applying aspin current to orient a magnetization of the free layer to provide alow or high resistance state, and after orienting the magnetization ofthe free layer, removing the voltage and the spin current.

Additional embodiments of magnetic tunnel junction cells and memorydevices are disclosed, as well methods of making and using the cells.These and various other features and advantages will be apparent from areading of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing detailed description of various embodiments of the disclosurein connection with the accompanying drawings, in which:

FIG. 1 is a cross-sectional schematic diagram of an illustrativemagnetic tunnel junction cell with in-plane magnetization orientation;FIG. 1A is a cross-sectional schematic diagram of an illustrativeperpendicular anisotropy magnetic tunnel junction cell with out-of-planemagnetization orientation;

FIG. 2 is a cross-sectional schematic diagram of a perpendicularanisotropy magnetic tunnel junction cell having a stress-assisted switchwith the switch in the “off” state;

FIG. 3 is a schematic diagram of a memory device including the magnetictunnel junction cell of FIG. 2;

FIG. 4 is a cross-sectional schematic diagram of the magnetic tunneljunction cell of FIG. 2 with a stress being applied to change thephase-change material from its antiferromagnetic state to itsferromagnetic state;

FIG. 5A is a cross-sectional schematic diagram of the magnetic tunneljunction cell of FIG. 2 with a switching current applied in a firstdirection thereto; FIG. 5B is a cross-sectional schematic diagram of themagnetic tunnel junction cell of FIG. 2 with a switching current appliedin a second direction thereto;

FIG. 6 is a flow chart of a method for forming a magnetic tunneljunction cell, such as the cell of FIG. 2; and

FIG. 7 is a flow chart of a method for using a memory device, such asthe memory device of FIG. 3.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

This disclosure is directed to spin-transfer torque memory, alsoreferred to as spin torque memory, spin torque RAM, or STRAM, and themagnetic tunnel junction cells (MTJs) that are a part of the memory. Thespin magnetic tunnel junction cells (MTJs) of this disclosure utilize amechanical strain to assist in the switching of the magnetizationorientation of the free layer of the magnetic tunnel junction cell.Nano-mechanical tensile stress is applied to a phase-change materiallayer within the magnetic tunnel junction cell to increase the latticeparameters to activate the phase change and orient the layermagnetization.

In the following description, reference is made to the accompanying setof drawings that forms a part hereof and in which are shown by way ofillustration several specific embodiments. It is to be understood thatother embodiments are contemplated and may be made without departingfrom the scope or spirit of the present disclosure. The followingdetailed description, therefore, is not to be taken in a limiting sense.The definitions and descriptions provided herein are to facilitateunderstanding of certain terms used frequently herein and are not meantto limit the scope of the present disclosure.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the specification and attached claims areapproximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” encompass embodiments having pluralreferents, unless the content clearly dictates otherwise. As used inthis specification and the appended claims, the term “or” is generallyemployed in its sense including “and/or” unless the content clearlydictates otherwise.

While the present disclosure is not so limited, an appreciation ofvarious aspects of the disclosure and of the invention will be gainedthrough a discussion of the Figures and the examples provided below.

FIG. 1 is a cross-sectional schematic diagram of a magnetic tunneljunction cell 10 that includes a soft ferromagnetic free layer 12 and aferromagnetic reference (i.e., pinned) layer 14. Ferromagnetic freelayer 12 and ferromagnetic pinned layer 14 are separated by an oxidebarrier layer 13 or non-magnetic tunnel barrier. Note that other layers,such as seed or capping layers, are not depicted for clarity.

Ferromagnetic layers 12, 14 may be made of any useful ferromagnetic (FM)material such as, for example, Fe, Co or Ni and alloys thereof, such asNiFe and CoFe. Ternary alloys, such as CoFeB, may be particularly usefulbecause of their lower moment and high polarization ratio, which aredesirable for the spin-current switch. Either or both of free layer 12and pinned layer 14 may be either a single layer or a syntheticantiferromagnetic (SAF) coupled structure, i.e., two ferromagneticsublayers separated by a metallic spacer, such as Ru or Cu, with themagnetization orientations of the sublayers in opposite directions toprovide a net magnetization. The magnetization orientation offerromagnetic free layer 12 is more readily switchable than themagnetization orientation of ferromagnetic pinned layer 14. Barrierlayer 13 may be made of an electrically insulating material such as, forexample an oxide material (e.g., Al₂O₃, TiO_(x) or MgO). Other suitablematerials may also be used. Barrier layer 13 could optionally bepatterned with free layer 12 or with pinned layer 14, depending onprocess feasibility and device reliability.

The following are various specific examples of magnetic tunnel junctioncells 10. In some embodiments of magnetic tunnel junction cell 10, oxidebarrier layer 13 includes Ta₂O₅ (for example, at a thickness of about0.5 to 1 nanometer) and ferromagnetic free layer 12 and a ferromagneticpinned layer 14 include NiFe, CoFe, or Co. In other embodiments ofmagnetic tunnel junction cell 10, barrier layer 13 includes GaAs (forexample, at a thickness of about 5 to 15 nanometers) and ferromagneticfree layer 12 and ferromagnetic pinned layer 14 include Fe. In yet otherembodiments of magnetic tunnel junction cell 10, barrier layer 13includes Al₂O₃ (for example, a few (e.g., about 1-5) nanometers thick)and ferromagnetic free layer 12 and ferromagnetic pinned layer 14include NiFe, CoFe, or Co.

A first electrode 18 is in electrical contact with ferromagnetic freelayer 12 and a second electrode 19 is in electrical contact withferromagnetic pinned layer 14. Electrodes 18, 19 electrically connectferromagnetic layers 12, 14 to a control circuit providing read andwrite currents through layers 12, 14. The resistance across magnetictunnel junction cell 10 is determined by the relative orientation of themagnetization vectors or magnetization orientations of ferromagneticlayers 12, 14. The magnetization direction of ferromagnetic pinned layer14 is pinned in a predetermined direction while the magnetizationdirection of ferromagnetic free layer 12 is free to rotate under theinfluence of spin torque. Pinning of ferromagnetic pinned layer 14 maybe achieved through, e.g., the use of exchange bias with anantiferromagnetically ordered material such as PtMn, IrMn, and others.

In some embodiments, magnetic tunnel junction cell 10 is in the lowresistance state where the magnetization orientation of ferromagneticfree layer 12 is parallel and in the same direction of the magnetizationorientation of ferromagnetic pinned layer 14. This is termed the lowresistance state or “0” data state. In other embodiments, magnetictunnel junction cell 10 is in the high resistance state where themagnetization orientation of ferromagnetic free layer 12 isanti-parallel and in the opposite direction of the magnetizationorientation of ferromagnetic pinned layer 14. This is termed the highresistance state or “1” data state.

Switching the resistance state and hence the data state of magnetictunnel junction cell 10 via spin-transfer occurs when a current, passingthrough a magnetic layer of magnetic tunnel junction cell 10, becomesspin polarized and imparts a spin torque on free layer 12 of magnetictunnel junction cell 10. When a sufficient spin torque is applied tofree layer 12, the magnetization orientation of free layer 12 can beswitched between two opposite directions and accordingly, magnetictunnel junction cell 10 can be switched between the parallel state(i.e., low resistance state or “0” data state) and anti-parallel state(i.e., high resistance state or “1” data state).

Free layer 12 is where data or bit information is stored when the deviceoperates under “read”, or overwritten when the device operates under“write”. Each ferromagnetic layer 12, 14 acts as “spin filter” when cell10 writes with “0” or “1” as the switching current passes through inopposite directions to alter magnetization of free layer 12.

The magnetization orientations of free layer 12 and pinned layer 14 ofmagnetic tunnel junction cell 10 are in the plane of the layers, orin-plane. FIG. 1A illustrates an alternate embodiment of a magnetictunnel junction cell that has the magnetization orientations of the freelayer and the pinned layer perpendicular to the plane of the layers, orout-of-plane.

Similar to magnetic tunnel junction cell 10 of FIG. 1, magnetic tunneljunction cell 10A of FIG. 1A has soft ferromagnetic free layer 12A and aferromagnetic reference (i.e., pinned) layer 14A separated by an oxidebarrier layer 13A or non-magnetic tunnel barrier. A first electrode 18Ais in electrical contact with ferromagnetic free layer 12A and a secondelectrode 19A is in electrical contact with ferromagnetic pinned layer14A. Other layers, such as seed or capping layers, are not depicted forclarity. Electrodes 18A, 19A electrically connect ferromagnetic layers12A, 14A to a control circuit providing read and write currents throughlayers 12A, 14A. The various elements of cell 10A are similar to theelement of cell 10, described above, except that the magnetizationorientations of layers 12A, 14A are oriented perpendicular to the layerextension rather than in the layer plane.

Free layer 12A and pinned layer 14A each have a magnetizationorientation associated therewith, illustrated in FIG. 1A. In someembodiments, magnetic tunnel junction cell 10A is in the low resistancestate or “0” data state where the magnetization orientation of freelayer 12A is in the same direction of the magnetization orientation ofpinned layer 14A. In other embodiments, magnetic tunnel junction cell10A is in the high resistance state or “1” data state where themagnetization orientation of free layer 12A is in the opposite directionof the magnetization orientation of pinned layer 14A.

Similar to cell 10 of FIG. 1, switching the resistance state and hencethe data state of magnetic tunnel junction cell 10A via spin-transferoccurs when a current, passing through a magnetic layer of magnetictunnel junction cell 10A, becomes spin polarized and imparts a spintorque on free layer 12A. When a sufficient spin torque is applied tofree layer 12A, the magnetization orientation of free layer 12A can beswitched between two opposite directions and accordingly, magnetictunnel junction cell 10A can be switched between the low resistancestate or “0” data state and the high resistance state or “1” data state.

In accordance with this disclosure, the switching of the free layermagnetization orientation is facilitated by a coupling layer proximatethe free layer. The coupling layer has a phase change material, which,upon its phase change, destabilizes the free layer and reduces theswitching current needed. The phase change material of the couplinglayer is incited to phase change by mechanical stress or strain.

A perpendicular magnetic tunnel junction cell structure thatincorporates a strain-assisted coupling layer is illustrated in FIG. 2as magnetic tunnel junction cell 20. Magnetic tunnel junction cell 20includes a soft perpendicular ferromagnetic free layer 22 and aperpendicular ferromagnetic reference (i.e., pinned) layer 24.Ferromagnetic free layer 22 and ferromagnetic pinned layer 24 areseparated by an oxide barrier layer 23 or non-magnetic tunnel barrier.The magnetization orientation of layer 22, 24 is perpendicular to thelayer, or, out-of-plane. Non-limiting examples of suitable materials forthese layers include: for free layer 22, a thin layer (e.g., about 2-30nm) of Co/Pt multilayers or FePt alloys or CoFe/Pt or CoFeX where X is arare-earth transition metal such as Tb or Gd; for pinned layer 24, athick layer (e.g., about 5-50 nm) of Co/Pt multilayers or FePt alloys orCoFePt or CoFeX; for barrier 23, insulating material (e.g., about 10-30Angstroms) such as Al₂O₃ or MgO. Alternately, free layer 22, pinnedlayer 24 and barrier layer 23 could be any of the materials describedabove in relation to free layer 12, 12A, pinned layer 14, 14A or barrierlayer 13, 13A.

Unlike magnetic tunnel junction cells 10, 10A of FIGS. 1 and 1A,magnetic tunnel junction cell 20 also includes a second ferromagneticpinned layer 26 and a phase change material coupling layer 25 positionedbetween free layer 22 and second pinned layer 26. Coupling layer 25 maybe adjacent to one or both of free layer 22 and pinned layer 26 or mayhave an intermediate layer therebetween. Second pinned layer 26 can haveproperties similar to pinned layer 24, or any of the properties orcharacteristics discussed above in relation to pinned layer 14 of cell10 or pinned layer 14A of cell 10A. In some embodiments, second pinnedlayer 26 and coupling layer 25 have magnetization orientations that arein the plane of the layers, or, in-plane. Coupling layer 25 is formed ofa phase change material, which changes a physical property upon anactivating incident, such as being exposed to a voltage. Coupling layer25 may be formed from an antiferromagnetic or superparamagnetic phasetransition material that can change to a ferromagnetic material upon anactivating incident. A non-limiting example of a suitable phase changematerial that transitions from magnetic to antimagnetic and back is FeRhand ternary alloys thereof, such as FeRhIr and FeRhPt. Second pinnedlayer 26 provides directional pinning of coupling layer 25 when couplinglayer 25 is in its magnetic state.

Magnetic tunnel junction cell 20 has a first electrode 28 in electricalcontact with second ferromagnetic pinned layer 26 and a second electrode29 in electrical contact with ferromagnetic pinned layer 24. Electrodes28, 29 are formed of an electrically conducting material, typicallymetal. An example of a suitable metal for electrodes 28, 29 is Pt.Electrodes 28, 29 electrically connect ferromagnetic layers 22, 24, 26and coupling layer 25 to a control circuit.

The illustrative spin-transfer torque magnetic tunnel junction cell 20may be used to construct a memory device where a data bit is stored inthe magnetic tunnel junction cell by the relative magnetization state offree layer 22 with respect to pinned layer 24. The stored data bit canbe read out by measuring the resistance of cell 20 which changes withthe magnetization direction of free layer 22 relative to pinned layer24. FIG. 3 illustrates magnetic tunnel junction cell 20 incorporatedinto a memory device 30 with a transistor and control circuit.

Tunnel junction cell 20, having free layer 22, barrier 23, pinned layers24, 26 and coupling layer 25, is connected to bit line BL via electrode28 and to word line WL via electrode 29 and transistor 33.

Proximate tunnel junction cell 20 is at least one actuator electrode 31,in this embodiment, first and second actuator electrodes 31A, 31B.Electrode 31, e.g., actuator electrodes 31A, 31B, is present proximateat least coupling layer 25 and optionally proximate one or more of freelayer 22, pinned layer 24, and second pinned layer 26. Electrode(s) 31may be formed of a piezoelectric material or a magnetoelectric material.An example of a suitable piezoelectric material for electrode(s) 31 islead zirconate titanate (PbZrTiO). Actuator electrode(s) 31 areconnected to receive a voltage therethrough, for example, from a timingcircuit control 35. The voltage through electrode(s) 31 may becoincident with write voltage passed through cell 20 to write or switchfree layer 22.

Actuator electrode(s) 31 initiate stress and strain that is relayed ortransferred to coupling layer 25. For example, as voltage is applied toelectrode(s) 31, the voltage induces a nano-mechanical strain inelectrode(s) 31 which transfers to the proximate coupling layer 25. Thisstrain on the phase change material of coupling layer 25 increases thelattice parameters of the material to activate a switch change from itsantiferromagnetic (AF) state to ferromagnetic (F) state in the appliedstress direction.

Using FeRh as an example phase change material for coupling layer 25,the AF-F state transition occurs when the FeRh lattice constant changes(under increasing temperature) about 0.3%-0.5%. With FeRh having aYoung's modulus ε_(FeRh)=1.7×10¹¹ Pa, only approximately a few volts orless are needed to generate the 0.3%-0.5% strain level. At such avoltage level, the actuation voltage source can be shared with theaddress signal to synchronize the “write” or switching event through asimple RC delay circuitry; such as timing circuit control 35.

Spin torque switching current requirement on a device with perpendicularanisotropy magnetic layers, such as magnetic tunnel junction cell 10A ofFIG. 1A, is:

$I_{d} = {\frac{\alpha\;{eM}_{s}V}{\hslash\; g}\left\lbrack {H_{k} - H_{eff}} \right\rbrack}$H_(eff) = H_(ex) + H + 4π M_(s)where M_(s) and H_(k) are respectively magnetization and anisotropyfield of the free layer, and H is the perpendicular field. When theout-of-plane field is at zero, H=0 and the required threshold switchingcurrent is:

$I_{d} = {\frac{\alpha\;{eM}_{s}V}{\hslash\; g}\left\lbrack {H_{k} - {4\pi\; M_{s}}} \right\rbrack}$

When there is no stress applied (H_(ex)=0), the switch signal is “OFF”.The H_(eff) needed to overcome the anisotropy field minusdemagnetization field is usually in the order of 10 KOe. However, in thedesign of this disclosure, such as magnetic tunnel junction cell 20,H_(eff) also includes the exchange field from the phase change material(e.g., FeRh) in the ferromagnetic state. H_(ex) is thus determined by:

$H_{ex} = {\frac{\sigma}{M_{s}\delta_{SL}} = \frac{2\sqrt{AK}}{M_{s}\delta_{SL}}}$where δ_(SL) is the free layer thickness of free layer 22, A is theinterlayer exchange constant and K is the anisotropy constant. With thehorizontal exchange field H_(ex) in the phase change material couplinglayer 25 generated from the strain applied to it, the required switchingfield H_(eff) (with the presence of the exchange field from the phasechange material) can be reduced down to 10% of H_(eff) (without thephase change material). Therefore, the switching current for magnetictunnel junction cell 20 will be approximately only 10% of the switchingcurrent for magnetic tunnel junction cell 10A.

Although only 10% of the current is needed to switch free layer 22 ofcell 20 as compared to free layer 12A of cell 10A, a voltage is neededfor actuator electrode(s) 31 to induce the magnetic phase transition ofcoupling layer 25. The voltage is determined by:

$V = {\frac{\Delta\; L\; ɛ\;{DS}_{33}^{E}}{{Ld}_{33}} = {\sigma\;\frac{ɛ\;{DS}_{33}^{E}}{d_{33}}}}$

Applying the following parameters, ΔL/L=0.5%, D=50 nm, S₃₃=23×10⁻¹²m²/N, d₃₃=220×10⁻¹² C/m (which is standard for a sol-gel PZT material),and ε=1.8×10¹¹Nm⁻², the resulting voltage is V=4.7 V.

Based on at least the discussion above and the theory behind it, adesign such as memory device 30, with magnetic tunnel junction cell 20having a phase change coupling layer 25 and actuator electrode(s) 31,has numerous design advantages over memory devices that have aconventional magnetic tunnel junction cell or that have a magnetictunnel junction cell that utilizes other methods to facilitate switchingof the free layer. The switching current in needed to switch the freelayer (unstabilized by the coupling layer) is less than about 10% ofthat needed when no free layer instability is present. Additionally,other advantages exist. For example, by using a coupling layer that hasa phase change that is strain activated, there is no need to heat thecoupling layer or the free layer, so that the design is more thermallyreliable than other designs. Because the AF-F switching is based on anapplied voltage, the AF-F switch and the subsequent free layer switchingcan be precisely controlled. This AF-F switching is fast and efficient,with the transition from AF-F and from F-AF being in the range of femtoseconds, with no hysteresis.

Referring again to the figures, particularly to FIG. 2 and to FIGS. 4and 5A and 5B, the operation mechanism of cell 20, in particular theswitching process of the magnetization orientation of free layer 22 is a2-step process.

As illustrated in FIG. 2, the phase change material of coupling layer 25is initially in its antiferromagnetic state, with an undefinedmagnetization orientation in relation to second pinned layer 26. Bothpinned layer 24 and free layer 22 have a perpendicular or out-of-planemagnetization orientation, with the magnetization of free layer 22 beingundefined as to orientation (e.g., either the same or opposite directionand the magnetization orientation of pinned layer 24). An interfaceexists between free layer 22 and coupling layer 25. Free layer 22 has athermal stability of about K_(u)V=0.5 M_(s)V(H_(k)−4πM_(s)) where Ku isthe anisotropy constant.

To define the data state of cell 20, either as a “0” with themagnetization orientations of free layer 22 and pinned layer 24 in thesame direction, or as a “1” with the magnetization orientations of freelayer 22 and pinned layer 24 in the opposite directions, the voltage toactuator electrode(s) 31 (in FIG. 3) is switched “ON”. The voltage inelectrode(s) 31 causes a stress or strain in electrode(s) 31, which isapplied to coupling layer 25. The stress on coupling layer 25 causes thephase change material to switch from its antiferromagnetic (AF) stage toits ferromagnetic (F) state and orient itself with the magnetizationorientation of second pinning layer 26, illustrated in FIG. 4. Theexchange field between free layer 22 and coupling layer 25 reduces thestability of free layer 22; the instability in free layer 22 shown inFIG. 4.

In FIG. 5A, with free layer 22 destabilized, spin current I is passedthrough magnetic tunnel junction cell 20 via electrodes 28, 29 (FIG. 2)in the direction from free layer 22 to pinned layer 24. This results inan orientation of the magnetization of free layer 22 with the current,but opposite to the magnetization orientation of pinned layer 24,writing the data state “1”. The spin current is removed, as is thevoltage to electrode(s) 31. The resulting free layer stability isrestored to the original thermal stability, approximately0.5M_(s)V(H_(k)−4πM_(s)), and the resulting magnetization orientation isretained.

In FIG. 5B, with free layer 22 destabilized due to voltage onelectrode(s) 31, spin current I is passed through magnetic tunneljunction cell 20 via electrodes 28, 29 (FIG. 2) in the direction frompinned layer 24 to free layer 22. This results in an orientation of themagnetization of free layer 22 with the current and with themagnetization orientation of pinned layer 24, writing the data state“0”. The spin current is removed, as is the voltage to electrode(s) 31.The resulting free layer stability is restored to its original thermalstability, approximately 0.5M_(s)V(H_(k)−4πM_(s)), and the resultingmagnetization orientation is retained.

The magnetic tunnel junction cell (e.g., cell 20) and memory structures(e.g., memory device 30) of this disclosure may be made by well-knownthin film building and removal techniques such as chemical vapordeposition (CVD), physical vapor deposition (PVD), atomic layerdeposition (ALD), photolithography, dry etching, wet etching, or ionmilling. The magnetization orientations of the pinned layer(s) (e.g.,layers 24, 26) may be set immediately after forming the pinned layer orafter forming subsequent layer(s). The actuator electrode(s) may beformed using well-known thin film techniques or may be previously formedand connected to the cell.

FIG. 6 illustrates stepwise a method for making magnetic tunnel junction20 or other magnetic tunnel junction cell that utilizes a mechanicalstrain to assist in the switching of the magnetization orientation ofthe free layer. Method 100 includes Step 101 of forming a magnetictunnel junction having a phase change coupling layer proximate the freelayer (e.g., forming cell 20 having coupling layer 25 proximate freelayer 22). Step 103 of the method includes providing at least oneactuator electrode proximate at least the coupling layer (e.g.,providing actuator electrode(s) 31 proximate at least coupling layer25). In Step 105, the actuator electrode(s) are electrically connectedto a voltage source to apply voltage through the electrodes.

FIG. 7 illustrates stepwise a method for writing a data state tomagnetic tunnel junction cell 20 or other magnetic tunnel junction cellthat utilizes a mechanical strain to assist in the switching of themagnetization orientation of the free layer. Nano-mechanical tensilestress is applied to a phase-change material layer within the magnetictunnel junction cell to increase the lattice parameters to activate thephase change and orient the layer magnetization.

Method 110 includes starting at 111 with a magnetic tunnel junction cell(e.g., cell 20) with its phase change coupling layer (e.g., couplinglayer 25) in the antiferromagnetic state. In step 113, the voltage toactuator electrode(s) (e.g., electrode(s) 31 in FIG. 3) is switched ON.This causes a stress or strain in the electrode(s), which is applied tothe phase change coupling layer (e.g., coupling layer 25). The stress onthe coupling layer causes the phase change material to switch from itsantiferromagnetic (AF) stage to its ferromagnetic (F) state, in Step114A. The exchange field between the coupling layer and the proximatefree layer (e.g., free layer 22) reduces the stability of the free layerfacilitating orienting the magnetization of that layer; Step 114B. InStep 115, spin current is turned ON and passed through the magnetictunnel junction cell (e.g., cell 20). This current orients the freelayer magnetization (Step 116) to either the same direction or theopposite direction as of the corresponding pinned layer (e.g., pinnedlayer 24). In Step 117, the spin current is turned OFF, and the voltageto the electrode(s) is turned OFF. The resistance state of the magnetictunnel junction cell is determined in Step 119; the resistance statewill be either “0” or “1”.

Thus, embodiments of the MAGNETIC MEMORY WITH STRAIN-ASSISTED EXCHANGECOUPLING SWITCH are disclosed. The implementations described above andother implementations are within the scope of the following claims. Oneskilled in the art will appreciate that the present disclosure can bepracticed with embodiments other than those disclosed. The disclosedembodiments are presented for purposes of illustration and notlimitation, and the present invention is limited only by the claims thatfollow.

What is claimed is:
 1. A magnetic tunnel junction cell comprising: afirst ferromagnetic pinned layer, a ferromagnetic free layer, and anon-magnetic barrier layer therebetween, the first pinned layer and thefree layer each having an out-of-plane magnetization orientation, asecond ferromagnetic pinned layer and a coupling layer between thesecond pinned layer and the free layer, the coupling layer comprising aphase change material that facilitates switching of the ferromagneticfree layer.
 2. The magnetic tunnel junction of claim 1, wherein thephase change material of the coupling layer facilitates switching of theferromagnetic free layer upon its phase change.
 3. The magnetic tunneljunction of claim 1, wherein the phase change material comprisesantiferromagnetic material or superparamagnetic phase transitionmaterial.
 4. The magnetic tunnel junction of claim 1, wherein the phasechange material comprises FeRh or ternary alloys thereof.
 5. Themagnetic tunnel junction cell of claim 1, wherein the second pinnedlayer has an in-plane magnetization orientation.
 6. The magnetic tunneljunction cell of claim 1, wherein the coupling layer is adjacent to boththe free layer and the second pinned layer.
 7. The magnetic tunneljunction cell of claim 1 further comprising a first electrode and asecond electrode electrically connected to pass a current through atleast the free layer and the first pinned layer.
 8. The magnetic tunneljunction cell of claim 1 further comprising at least one actuatorelectrode proximate the coupling layer.
 9. The magnetic tunnel junctioncell of claim 8 wherein the at least one actuator electrode comprises apiezoelectric material.
 10. The magnetic tunnel junction cell of claim 9wherein the piezoelectric material comprises lead zirconate titanate.11. The magnetic tunnel junction of claim 1 further comprising a firstactuator electrode and a second actuator electrode.
 12. A memory devicecomprising: a magnetic tunnel junction cell comprising a firstferromagnetic pinned layer, a ferromagnetic free layer, and anon-magnetic barrier layer therebetween, the first pinned layer and thefree layer each having an out-of-plane magnetization orientation, themagnetic tunnel junction cell further comprising a second ferromagneticpinned layer and a coupling layer between the second pinned layer andthe free layer, the coupling layer comprising a phase change materialthat facilitates switching of the ferromagnetic free layer, and thesecond pinned layer has an in-plane magnetization orientation; a firstelectrode and a second electrode electrically connected to the magnetictunnel junction cell to pass a current therethrough; at least oneactuator electrode proximate the coupling layer; and a voltage sourceelectrically connected to the at least one actuator electrode.
 13. Thememory device of claim 12 wherein the voltage source, when activated,induces a nano-mechanical strain in the at least one actuator electrode.14. The memory device of claim 12 further comprising a timing circuitoperably connected to the voltage source.
 15. The memory device of claim12 wherein a write voltage passed through the first and secondelectrodes is coincident with a voltage through the at least oneactuator electrode.
 16. The magnetic tunnel junction of claim 12,wherein the phase change material comprises antiferromagnetic materialor superparamagnetic phase transition material.
 17. The magnetic tunneljunction of claim 16, wherein the phase change material comprises FeRhor ternary alloys thereof.
 18. A magnetic element comprising: a firstferromagnetic pinned layer, a ferromagnetic free layer, and anon-magnetic barrier layer therebetween, the first pinned layer and thefree layer each having an out-of-plane magnetization orientation, asecond ferromagnetic pinned layer and a coupling layer between thesecond pinned layer and the free layer, the coupling layer comprising aphase change material that facilitates switching of the ferromagneticfree layer, and the second pinned layer has an in-plane magnetizationorientation.
 19. The magnetic element of claim 18, wherein the couplinglayer comprises FeRh.
 20. The magnetic element of claim 18, wherein thecoupling layer is adjacent to both the free layer and the second pinnedlayer.